Passive pseudorandom noise (PRN) ranging systems such as the United States' Global Positioning System (GPS) and the Russian Global Navigation System (GLONASS) allow a user to precisely determine his latitude, longitude, elevation and time of day. PRN ranging system receivers typically accomplish this by using time difference of arrival and Doppler measurement techniques on precisely-timed signals transmitted by orbiting satellites. Because only the satellites transmit, the need for two-way communications is avoided, and an infinite number of receivers may thus be served simultaneously.
In order for the receivers to extract the requisite information, the signals transmitted by the satellites must contain certain information. For example, within the GPS system, each carrier signal is modulated with low frequency (typically 50 Hz) digital data which indicates the satellite's ephemeris (i.e. position), current time of day (typically a standardized time, such as Greenwich Mean Time), and system status information.
Each carrier is further modulated with one or more unique, high frequency pseudorandom noise (PRN) codes, which provide a mechanism to precisely determine the signal transmission time from each satellite. Different types of PRN codes are used for different system applications. For example, within the GPS system, a so-called low-frequency "C/A code" is used for low cost, less accurate commercial applications, and a higher-frequency "P-code" is used for higher accuracy military applications.
Thus, a typical PRN receiver receives a composite signal consisting of one or more of the signals transmitted by the satellites within view, that is within a direct line-of-sight, as well as noise and any interfering signals. The composite signal is first fed to a downconverter which amplifies and filters the incoming composite signal, mixes it with a locally generated carrier reference signal, and thus produces a composite intermediate frequency (IF) signal. A decoder or channel circuit then correlates the composite signal by multiplying it by a locally generated version of the PRN code signal assigned to a particular satellite of interest. If the locally generated PRN code signal is properly timed, the digital data from that particular satellite is then properly detected.
Because the signals transmitted by different satellites use unique PRN codes and/or unique carrier frequencies, the receiver signals from different satellites are automatically separated by the multiplying process, as long as the locally generated PRN code has the proper timing. A delay lock loop (DLL) tracking system which correlates early, punctual, and late versions of the locally generated PRN code signal against the received composite signal is also typically used to maintain PRN code lock in each channel. The receiver's three dimensional position, velocity and precise time of day is then calculated by using the PRN code phase information to precisely determine the transmission time from at least four satellites, and by detecting each satellite's ephemeris and time of day data.
For more information on the format of the GPS CDMA system signals, see "Interface Control Document ICD-GPS-200, Sep. 26, 1984", published by Rockwell International Corporation, Satellite Systems Division, Downey, Calif. 90241.
For more information on the format of the GLONASS system signals, see "The GLONASS System Technical Characteristics and Performance", Working Paper, Special Committee on Future Air Navigation Systems (FANS), International Civil Aviation Organization (ICAO), Fourth Meeting, Montreal, Quebec, Canada, 2-20 May 1988.
A number of problems face the designer of PRN receivers. One problem concerns accurate phase and frequency tracking of the received signals; another problem concerns the correction of relative divergence between the received signals and the local PRN code signal generators in the presence of ionospheric distortion. In addition, because GPS systems depend upon direct line of sight for communication propagation, any multipath fading can further distort received signal timing estimates.
Certain GPS system designers have realized that the tracking error caused by multipath distortion in the out-of-phase condition can be reduced by narrowing the delay spacing between the early and late correlators in the DLL. However, this has heretofore not been thought to be advantageous under a wide range of operating conditions, since the DLL is then more susceptible to loss of lock due to sudden dynamic motions of the receiver. See, for example, Hagerman, L. L., "Effects of Multipath on Coherent and Non-coherent PRN Ranging Receiver", Aerospace Corporation Report No. TOR-0073(3020-03)-3, 15 May 1973.
As a result, most present-day PRN receivers use a DLL time-delay spacing of one PRN code bit (or chip) time. Historically, there have been several reasons for this adherence to one chip-time spacing.
For example, early PRN receivers were invariably of the P-code, or high frequency variety. Since P-code chip time is relatively narrow as compared with the correlator DLL spacing, it was feared that Doppler and random noise considerations would cause loss of PRN code lock if the correlator spacing was made any narrower.
Furthermore, narrower correlator spacing is not particularly desirable, as it increases the time required to lock onto a given PRN signal. This is of particular concern in PRN ranging systems, where often times many codes and code delays must be tried.
Finally, it has been thought that because a narrowed correlator spacing requires a higher precorrelation bandwidth, the resulting higher sampling rates and higher digital signal processing rates were not justified.
What is needed is a way to reduce the tracking errors present in PRN ranging receivers, especially those of the lower-frequency C/A code type, in the presence of multipath fading, without degrading the signal acquisition capability of the receiver, or increasing errors due to Doppler shift, sudden receiver motion, or other noise sources.